This invention relates to a temperature indicating device. More particularly, it relates to a temperature indicating device which utilizes cavities filled with temperature sensitive materials which must be visibly sealed from their surrounding environment.
A temperature indicating device utilizing a thermally conductive foil as a carrier layer having a plurality of cavities formed by depressions in the material, each cavity being filled with mixtures of ortho-chloronitrobenzene, ortho-bromonitrobenzene and a dyestuff are described in copending U.S. patent application Ser. No. 896,115 filed Apr. 13, 1978 and assigned to Akzona Incorporated, assignee of this application. This temperature indicating device has never been applied in practice because no suitable adhesive agent was available with which the carrier layer and the cover layer could be bonded together without premature discoloration of the indicator layer. From the U.S. Pat. No. 3,002,385 temperature indicating devices are known in which use is made of a low-temperature bonding adhesive, such as an epoxy resin or a pressure-sensitive adhesive substance having a basis of silicones. For various reasons, such as insufficient resistance to the chemicals used, release of low-molecular constituents, which detrimentally affects durability, temperature indication and reproducibility, and the fact that they are insufficiently impervious to chemicals, said adhesives have been found to be unsuitable.
Owing to the lack of a suitable adhesive recourse was made in practice to the use of special constructions in which a shield was positioned between the fusible solid mixture and the indicator layer, which shield is removed just prior to use of the temperature indicating device. Such a construction is described in U.S. Pat. No. 3,677,088. Although such a temperature indicating device can very well be used, the above described shield causes production and use thereof to be complicated and expensive.
It has now been found that by choosing a special type of adhesive layer there is no longer need for the use of a shield.